With the advancement of information technology, the importance of display devices as information transfer media is increasing. In the field of display devices, requirements for large size, light weight, thin thickness, high image quality, etc. are gradually increasing in accordance with the advancement of technology. In order to satisfy these requirements, liquid crystal displays (LCDs) are widely used instead of conventional cathode ray tubes (CRTs).
A liquid crystal display device generally includes a substrate having pixels formed in a matrix, an opposing substrate, and a liquid crystal material with an anisotropic dielectric constant injected between the two substrates. In the liquid crystal display device, an electric field is applied between the two substrates, and the intensity of the applied electric field is adjusted, so that the amount of light passing through the liquid crystal material is controlled, thereby displaying a desired image. If a voltage is applied to liquid crystals through pixel electrodes provided to the respective pixels of the liquid crystal display device, alignment of the liquid crystals is changed accordingly, and diffraction is caused while light passes through the liquid crystals whose alignment is changed, thereby obtaining a desired image. Since a liquid crystal display device is not a self-luminescent display device, the liquid crystal display device is configured such that a lamp mounted on the back of the liquid crystal display device operates as a light source.
Generally, a liquid crystal display device has a plurality of lamps. In the liquid crystal display device, image signals which take up an entire screen during a single frame period are sequentially inputted and displayed on the screen when the screen is changed. At this time, an appropriate response time for realigning the liquid crystals is required to change the image according to a change in the inputted image signals, and hence there may be a degradation of image quality, since an afterimage of the previous screen may remain on a newly displayed screen when the liquid crystals are being realigned. A liquid crystal display device having a plurality of lamps may employ a scanning method to divide the plurality of lamps into a plurality of lamp groups and to control the lamp groups to be sequentially turned on, in order to become more energy efficiency and to prevent the degradation of display quality. When the scanning method is employed, the luminance of a display image may be increased by partially overlapping turned-on periods of neighboring lamp groups with one another. When the lamps are divided into two or more lamp groups and the driving of each of the lamp groups is controlled according to the scanning method, the liquid crystal display device may include an inverter circuit for controlling driving of each of the lamp groups.
A turned-on lamp may generate parasitic capacitance between the turned-on lamp and another turned-on lamp adjacent thereto on a panel. When driving is controlled for each of the lamp groups using a scanning method, particularly in a control method where turned-on periods of neighboring lamp groups partially overlap one another, only one lamp group may be turned on, or two or more neighboring lamp groups may be turned on together, depending on the time period. Therefore, during a time period in which only one lamp group is turned on, parasitic capacitance may be generated by turned-on lamps belonging to a corresponding lamp group. Alternatively, during the time period in which two or more lamp groups are turned on together, parasitic capacitance may be generated by turned-on lamps belonging to the corresponding two or more lamp groups. Accordingly, the parasitic capacitance generated by the turned-on lamps in the liquid crystal display device changes depending on the time period in which only one lamp group is turned on or two or more lamp groups are turned on.
In relation to this, the operation of the inverter circuits for controlling driving of the respective lamp groups will be described. The inverter circuit converts a DC voltage supplied from outside the liquid crystal display device into an appropriate AC voltage based on a corresponding driving control signal received from the outside, i.e., a signal for controlling turn-on of a corresponding lamp group. Then, the inverter circuit transforms the converted AC voltage into an appropriate operation voltage and supplies the transformed operation voltage to the corresponding lamp group. The operation voltage outputted from the inverter circuit and supplied to the lamps belonging to the corresponding lamp group is determined based on a resonance frequency and an operation frequency of the inverter circuit. Generally, the operation frequency is an arbitrarily fixed value determined when designing the inverter circuit, and the resonance frequency (fr) becomes 1/(2π√{square root over (LC)}). While L is a fixed value determined based on a physical structure of the inverter circuit, C may be change in value depending on a change in the parasitic capacitance generated by the turned-on lamps described above. Therefore, a change in the entire parasitic capacitance of the liquid crystal display device generated depending on the change in the number of turned-on lamp groups causes a change in the resonance frequency (fr), and the change in the resonance frequency (fr) generated while the operation frequency is fixed causes a change in the output operation voltage of the inverter circuit. In other words, the change in the entire parasitic capacitance of the liquid crystal display device caused when the number of turned-on lamp groups is changed in each time period causes a change in the resonance frequency (fr) of the inverter circuit for controlling turn-on of each of the lamp groups, and accordingly, a lamp driving voltage outputted from the inverter circuit is changed. The operation voltage change depends on the change in the resonance frequency (fr) which causes distortion of a current waveform supplied to the lamps, and therefore, the screen may flicker.
Accordingly, development of a lamp driving method and apparatus of an liquid crystal display device that enables stable lamp driving regardless of a change in a parasitic capacitance generated by a change in the number of turned-on lamp groups is desired, so as to cope with a change in the resonance frequency of an inverter apparatus for controlling turn-on of the lamps in the liquid crystal display device.